The immune system is composed of several different cell types that are involved in multiple highly regulated processes and together generate immune responses that are effective in eliminating foreign proteins. Further, these same immune cells have been found to possess a self-awareness property by virtue of, inter alia, regulatory membrane proteins that regulate cell-to-cell interactions. Such communication is critical for the survival of such organisms, as these same proteins are suggested to be an important determinant of transplant engraftment. However, no in vivo system exists to determine the molecular aspects of human immune cell-to-cell interactions and its regulation. Such a system provides a source for assays in human hematopoietic and immune system related functions in vivo, identification of novel therapies and vaccines.